The invention relates to a method for operating a reagent metering system which meters a reagent into the exhaust duct of an internal combustion engine upstream of an SCR catalytic converter, and to a device for carrying out the method.
The invention furthermore relates to a control device, with the aid of which the reagent metering system is operated, and to a control device program product with a program code stored on a machine-readable storage medium, for carrying out the method.
To aftertreat the exhaust gas of an internal combustion engine, use can be made of selective catalytic reduction (SCR), with the aim of reducing NOx in the exhaust gas. In this connection, a defined quantity of a selectively acting reagent is metered into the exhaust duct of the internal combustion engine. The reagent can be ammonia which is acquired, for example, from a precursor in the form of a urea-water solution in the exhaust duct by hydrolysis.
Such a reagent metering system is known, for example, from laid-open application DE 196 07 073 A1. The urea-water solution is conveyed here through a line from a tank to a metering valve and is metered into an exhaust duct of an internal combustion engine upstream of an SCR catalytic converter, wherein the metering rate is defined by means of the metering valve.
In current reagent metering systems, as are known under the applicant's designation DENOXTRONIC, a pump sucks the urea-water solution out of a reagent tank and compresses said solution to the system pressure required for atomizing, of, for example, 3 to 9 bar. The metering rate of the reagent is coordinated to the maximum possible NOx reduction by taking into consideration, for example, current internal combustion engine data and catalytic converter data.
The urea-water solution which is customarily used and is defined in DIN standards has the property of freezing at approximately −11° C. The volumetric expansion of the urea-water solution that is associated with the freezing may lead to damage to the lines and to further components, such as, for example, pump or metering valve. Therefore, it can be provided, after shutting down the internal combustion engine or after switching off the reagent metering system, to suck the urea-water solution back from the reagent metering system, in particular from the metering valve, into the tank. The effect is therefore achieved that the reagent metering system can freeze at temperatures of −11° C. or therebelow without there being any concern about damage due to the volumetric expansion of the freezing urea-water solution.
Laid-open application DE 10 2011 076 429 describes such a reagent metering system in which a pump can be switched over from a forwards operation to a reverse operation such that the pump, in addition to the building up of the predetermined system pressure, additionally permits the reagent to be sucked back.
Purely in principle, it is possible, instead of a pump driven by a rotating electric motor, to provide a solenoid actuator, specifically a reciprocating pump, in which an armature surrounded by a solenoid carries out a reciprocating movement. Such reciprocating pumps are activated, for example, by a periodically repeating squarewave signal. An activation duration is set here within a period in such a manner that a complete stroke of the plunger-type armature takes place.
A disadvantage of such solenoid actuators or reciprocating pumps is the high generation of noise at the armature stop. Patent DE 10 2007 028 059 B4 therefore describes a reciprocating pump for delivering a liquid, in which an impact damper made of elastomer is provided for reducing the noise. In addition, kinetic energy of the reciprocating piston is absorbed by a resetting spring and hydraulic damping. The mechanical design of the reciprocating pump is correspondingly complicated. The result is high manufacturing costs. Furthermore, the mechanically heavily loaded impact damper is exposed to increased wear which reduces the lifespan of the reciprocating pump.
Laid-open application DE 10 2011 088 701 A1 discloses a method for monitoring the armature movement of a reciprocating solenoid pump, in particular in the delivery module of a reagent metering system. For this purpose, a local minimum value in the current characteristic through the solenoid of the reciprocating solenoid pump is sought and identified as the time of the armature striking against an armature stop. In order to ascertain the local minimum value, use can be made of the passage through zero of the first time derivative of the current characteristic.
Laid-open application DE 197 19 602 A1 describes an activation of an electromagnetic valve having an armature. The armature is moved from a closed position of the valve into an open position and is kept in the latter. The activation is configured in such a way that it outputs a comparatively high voltage until the armature is released, outputs a smaller voltage during the flight phase of the armature and, after the open position is reached, again outputs a high retaining voltage to the solenoid of the valve. According to one embodiment, switching times for switching over between the voltages can be ascertained depending on the time characteristic of the current through the solenoid. The current curve here shows the known characteristic dependencies on the movement of the armature. The reduced voltage during the flight phase leads to a reduced acceleration of the armature. A speed is thereby set which is of a size sufficient in order to ensure secure closing of the valve, but is low enough for a generation of noise when the armature impacts against the end stop to be reduced.
It is the object of the invention to provide a method and a device with which a reduction in the generation of noise of a reciprocating pump is obtained.